This invention relates to calorimetric differential thermal analyzers such as those described in U.S. Pat. Nos. 3,554,002 and 4,095,453 both assigned to E. I. duPont de Nemours and Company.
Differential thermal analyzers measure the difference between sample and reference material temperatures as their temperatures are varied. Differential scanning calorimeters (DSC) are differential thermal analyzers which give quantitative information about the samples by measuring heat flow to and from the samples. The calorimetric analyzers described in the two named patents use a heat flow type DSC cell. In this cell a thermoelectric disk, used to support both the sample and the reference material, serves as the major heat flow path for transferring heat to the sample and reference materials. The disk also is a common material of the differential thermocouple for measuring the difference in temperatures between the sample and reference materials. This thermoelectric disk is mounted inside a silver heating block having a silver lid. The sample and reference materials are placed in sealed metal pans so that the thermal environment is reproducible from run to run.
While instruments designed along these lines are extremely useful and have been very well received by the thermal analyst, they tend to suffer from their inability to run more than one sample at a time or to compare samples simultaneously in the same run. The construction and design of an instrument in which several samples can be run simultaneously is quite difficult because all test positions must be thermally equivalent. The reference should not bear any special relationship with any of the samples, and interchange between the sample and reference positions should result in a thermally equivalent structure. The presence of more than one sample can give rise to the serious problem of crosstalk, i.e., the appearance of a feature on the DSC curve for one sample caused by a thermal transition occurring at another sample position. This is highly undesirable and should be eliminated to the extent possible. To reduce crosstalk between samples, thermal communication between the positions of the samples or reference should be reduced to a low level.
Among the known multiple sample differential thermal analyzers (albeit not of the heat flow type), it has been customary to provide thermal isolation between each of the sample positions and the reference position. Such isolation may be accomplished by a thermal insulator such as a mica screen between each test position. Typical of these instruments is that described in the publication Chemical Communications by the University of Stockholm entitled "A Low Temperature DTA Apparatus for Simultaneous Measurement of Five Samples" by T. Horlin, T. Niklewski and M. Nygren, 1979 No. 9. This publication describes six symmetrically arranged holders, each thermally insulated from each other by means of a thin mica shield. However, this is not a heat flow type of instrument capable of giving quantitative information. Furthermore, since the effectiveness of all thermal insulation is relative, there is, in fact, crosstalk between the sample positions. Although tolerable in differential thermal analysis, significant crosstalk simply is not acceptable in differential scanning calorimetry.